Can you reference which major index fund has tanked 30% in one day?

To my knowledge Black Monday in 1982 was the biggest single day dip in the stock market, where the S&P 500 lost about 20%. See: https://en.m.wikipedia.org/wiki/List_of_largest_daily_change...

Also after these dips, did the market recover and how long did it take? Imo index funds are where you put your money away for decades. Over those years, what is the average gains per year?

There is nothing wrong with accounting for extreme events and that’s why all investment could be risky. But I am challenging your narrative.